Fix bug when checking if PLUGIN test is enabled

[rafaelwestphal]

Description

Current implementation doesn't work properly if the env variable is exported as an empty string.

Related issue

b/396679776

How has this been tested?

Created a go test sample to verify the behavior

Checklist:

• Unit tests
  • Unit tests do not apply.
  • Unit tests have been added/modified and passed for this PR.
• Integration tests
  • Integration tests do not apply.
  • Integration tests have been added/modified and passed for this PR.
• Documentation
  • This PR introduces no user visible changes.
  • This PR introduces user visible changes and the corresponding documentation change has been made.
• Minor version bump
  • This PR introduces no new features.
  • This PR introduces new features, and there is a separate PR to bump the minor version since the last release already.
  • This PR bumps the version.